Measurement of B(B- to D0 pi-) and B(barB0 to D+ pi-) and Isospin Analysis of B to D pi Decays

We present new measurements of branching fractions for the color-favored decays B^- --> D^0 pi^- and Bbar^0 --> D^+ pi^-. Using 9.67 x 10^6 BBbar pairs collected with the CLEO detector, we obtain the branching fractions BR(B^- --> D^0 pi^- =3D (49.7 +/- 1.2 +/- 2.9 +/- 2.2) x 10^{-4} and BR(Bbar^0 --> D^+ pi^- =3D (26.8 +/- 1.2 +/- 2.4 +/- 1.2) x 10^{-4}. The first error is statistical, the second is systematic, and the third is due to the experimental uncertainty on the production ratio of charged and neutral B mesons in Upsilon(4S) decays. These results, together with the current world average for the color-suppressed branching fraction BR(Bbar^0 --> D^0 pi^0), are used to determine the cosine of the strong phase difference delta_I between the I=1/2 and I=3/2 isospin amplitudes. We find cos(delta_I) = 0.863 (+0.024 -0.023) (+0.036 -0.035) (+0.038 -0.030), and obtain a 90% confidence interval of 16.5 degrees < delta_I < 38.1 degrees. This non-zero value of \delta_I strongly suggests the presence of final state interactions in the Dpi system.